pf: drain Ethernet rules cleanup before starting a new transaction

Inactive Ethernet rules get cleaned by a net_epoch callback. This
callback may still be pending when we try to start a new (pf rules)
transaction, causing it to fail.
This is especially likely to occur in scripted scenarios, such as the
regression tests.

Drain the epoch callbacks before starting a new transaction, ensuring
we've had the opportunity to clean up the inactive rules.

Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D34846

sys/netpfil/pf/pf_ioctl.c

@@ -5089,6 +5089,8 @@ DIOCCHANGEADDR_error:
 			free(ioes, M_TEMP);
 			break;
 		}
+		/* Ensure there's no more ethernet rules to clean up. */
+		epoch_drain_callbacks(net_epoch_preempt);
 		PF_RULES_WLOCK();
 		for (i = 0, ioe = ioes; i < io->size; i++, ioe++) {
 			ioe->anchor[sizeof(ioe->anchor) - 1] = '\0';